Nucleation speed

The nucleation temperature is decided by obtaining the exothermal peak temperature of crystallization measured by carrying out DTA analysis on each glass that is heat treated at various temperatures in advance (Figure 2-6 (a)), and by calculating the nucleation speed from the temperature difference with the crystallization temperature of glass that is not heat treated. Below... 

Figure 2-5 The temperature dependency of nucleation speed I and crystal growth speed U [(a) Vitrification occurs readily, (b) Crystals are formed readily] [Ref. 17].

Figure 2-6 DTA curves for non heat treated glass and for glass that is heat treated at various temperatures for 8 hours (a), and the relationship between heat treatment temperature and nucleation speed (b) [Ref. 18],...

In this way, the heat treatment temperature dependency of nucleation speed can be expressed with variations of [1/Tp - 1/T°p],... 

Fast crystallization allows faster molding cycles or clearer extruded thin film. Nucleation speeds this crystallization process. Crystallization is a complex subject because there can be a downside as well as immediate economic advantage. Many different types of chemicals can nucleate crystallization. The eff ect is specific for the polymer and the chemical. Glass fibers, pigments, talc, etc. can nucleate crystallization for some polymers. 

Correlations of nucleation rates with crystallizer variables have been developed for a variety of systems. Although the correlations are empirical, a mechanistic hypothesis regarding nucleation can be helpful in selecting operating variables for inclusion in the model. Two examples are (/) the effect of slurry circulation rate on nucleation has been used to develop a correlation for nucleation rate based on the tip speed of the impeller (16) and (2) the scaleup of nucleation kinetics for sodium chloride crystalliza tion provided an analysis of the role of mixing and mixer characteristics in contact nucleation (17). Pubhshed kinetic correlations have been reviewed through about 1979 (18). In a later section on population balances, simple power-law expressions are used to correlate nucleation rate data and describe the effect of nucleation on crystal size distribution. 

Equation (18-36) is the general expression for impeller-induced nucleation. In a fixed-geometry system in which only the speed of the circulating pump is changed and in which the flow is roughly proportional to the pump speed, Eq. (18-36) may be satisfactorily replaced with... 

The nucleation rate is plotted versus the supersaturation for different stirrer speeds in a log-log diagram (Figure 6.21). The kinetic order n in the correlating equation... 

The effect of nucleating sources is important when attempting to scale studies with models tO full-size components. A common practice in model tests is to expose the fluid to a high static pressure prior to testing in order to drive free gas into solution. In contrast the seeding of water tunnels is considered to improve scaling particularly in high-speed propeller applications . 

In precipitation reactions, powder characteristics depend on the speed of the nucleation of particles and their growth due to the mass flow to the surfaces. In freeze drying and spray drying, powder characteristics primarily depend on the size of droplets, which in turn is determined by the parameters of a nozzle and characteristics of the flow of a carrier gas. Both these methods enable one to obtain powders with very high surface area. 

Solid state reactions are also very common in producing oxide materials and are based on thermal treatment of solid oxides, hydroxides and metal salts (carbonates, oxalates, nitrates, sulphates, acetates, etc.) which decompose and react forming target products and evolving gaseous products. Solid-state chemistry states that, like in the case of precipitation, powder characteristics depend on the speed of the nucleation of particles and their growth however, these processes in solids are much slower than in liquids. 

Talsma et al. [1.34] described the freezing behavior of certain liposomes by DSC measurements. Besides the expected influences of freezing and rewarming speeds, and of the CPAs (mannitol and mannitol in Tris-buffer solutions) it was shown, that the heterogeneous and homogeneous crystallization in mannitol solutions exists and the nucleation of ice depends also on the liposome size In small liposomes (e. g. 0.14 pm) mannitol suppressed the heterogeneous crystallization more effectively than in large (0.87 pm) liposomes. If in certain substances no crystallization or eutectic mixtures can be found by DSC (cephalosporin, Williams [1.35]) with the used experimental conditions, one has to seek different conditions [1.32]. 

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